An electrical double layer capacitor is a charge accumulation device utilizing an electrical double layer formed in the interface of polarizable electrodes and an electrolytic solution.
When the electrolytic solution used in an electrical double layer capacitor possesses a low electric conductivity, the internal resistance of the electrical double layer capacitor increases thereby decreasing the voltage of the capacitor during charging and discharging. Therefore, an electrolytic solution used in an electrical double layer capacitor is required to possess a high electric conductivity.
Another requirement for an electrolytic solution is the capability of providing the capacitor using the electrolytic solution with a sufficiently large electrostatic capacity.
These characteristics are required to exhibit a small dependence on temperature. Specifically, the capacitor is required to maintain excellent electric conductivity and electrostatic capacity at a low temperature.
In addition to the above characteristics, the electrolytic solution is required to be durable over a long period of time.
When the electrolyte concentration of the electrolytic solution is low, the internal resistance of the capacitor increases during charging with a large current density due to an insufficient amount of ions. Therefore, the electrolytic solution preferably has an electrolyte concentration as high as possible.
However, because the electrolytes in the electrolytic solution tend to precipitate at a low temperature when the electrolyte concentration of an electrolytic solution is increased, the electrolyte concentration of the electrolytic solution must be maintained at a low level. This, however, decreases conductivity and impairs the above charging and discharging characteristics. In order to solve these problems, crystal precipitation of the electrolyte at a low temperature must be prevented by increasing the solubility of the electrolyte in the electrolytic solution. In addition, the electrolyte concentration of the electrolytic solution must be increased in order to overcome the above problems.
As an electrolytic solution conventionally used in electrical double layer capacitors, a solution comprising an electfrolyte such as a linear alkyl quartenary ammonium salt (e.g. tetraethyl ammonium salt) and a quartenary phosphonium salt dissolved in an aprotic solvent such as y-butyrolactone (hereinafter referred to as “GBL”) and propylene carbonate (hereinafter referred to as “PC”) can be given.
However, since these electrolytes have a low solubility in the organic solvent of about 0.7-1.5 mol/l, the electrolytic solution has a low conductivity, possibly giving rise to the previously mentioned problems of impaired charging and discharging characteristics.
In addition, a capacitor using an electrolytic solution comprising linear alkyl quartenary ammonium salt, quartenary phosphonium salt, or the like tends to have a low electrostatic capacity and a large internal resistance at a low temperature. Specifically, the capacitor also possesses a problem in regard to major change in the characteristics due to a temperature change.
Recently, an electrolytic solution comprising N,N′-dialkyl substituted imidazolium salt having a high solubility in the above organic solvent of about 3 mol/l as an electrolyte has been proposed (for example, Japanese Patent No. 2945890).
However, the electrolytic solution comprising N,N′-dialkyl substituted imidazolium salt as an electrolyte has a high viscosity and an unduly decreased conductivity at a low temperature as compared with the electrolytic solution comprising linear alkyl quartenary ammonium salt or quarternary phosphonium salt as an electrolyte. The electrical double layer capacitor manufactured using this electrolytic solution exhibits a problem in long term reliability. The electrostatic capacity is reduced greatly when the capacitor is subjected to a high voltage load over a long period of time.
In view of the above, the object of the present invention is to provide an electrolytic solution for an electrical double layer capacitor capable of dissolving an electrolyte in a high concentration, having excellent conductivity and ensuring excellent electrostatic capacity in a wide temperature range from a low temperature to a high temperature, possessing a high withstand voltage, and exhibiting excellent long term reliability with electrostatic capacity decreasing only with difficulty under a high voltage load for a long period of time. The present invention also provides an electrical double layer capacitor using the electrolytic solution.